The present invention relates to a disk drive which is an external memory device for a computer, and, more particularly, to a technique for preventing a plurality of disk drives in an array-type disk apparatus constituting a disk array from failing simultaneously and a technique for improving the host I/O response and improving the reliability at the time of data shifting among disk drives constituting a disk array group having a redundancy.
An array-type disk apparatus is one type of memory device systems which are to be connected to computers. The array-type disk apparatus is called a RAID (Redundant Arrays of Inexpensive Disks) and is a memory device which has a plurality of disk drives laid out in an array and a control section to control the disk drives. In the array-type disk apparatus, a read request (data read request) and a write request (data write request) are processed fast by the parallel operation of the disk drives and redundancy is added to data. As disclosed in Non-patent Publication 1 (“A Case for Redundant Arrays of Inexpensive Disks (RAID)”, David A. Patterson, Garth Gibson, and Randy H. Katz, Computer Science Division Department of Electrical Engineering and Computer Sciences, University of California Berkeley), array-type disk apparatuses are classified into five levels according to the type of redundant data to be added and the structure.
It is typical for array-type disk apparatuses available on the market that spare disk drives are mounted beforehand in the same array-type disk apparatus on the assumption that disk drives used may fail. In case where an array-type disk apparatus decides that a disk drive which is a member of the RAID of the array-type disk apparatus or a disk array group has failed, the array-type disk apparatus restores the same data and parity of the failed disk drive in the associated spare disk drive based on the data and parity of another disk drive. After restoration, the spare disk drive operates in place of the failed disk drive.
Further, if the data and parity of a disk drive are restored after the disk drive fails, an access is made to all the disk drives constituting the RAID group, lowering the on-line performance. As a solution to this problem, there is a technique which predicts a disk drive which is likely to fail, copies data in the paired spare disk drive before the disk drive fails and becomes inaccessible, and keeps the disk operation using the spare disk drive. Patent Document 1 (Japanese Patent Laid-Open No. 147112/1996) discloses a technique which copies data of a disk drive to its spare disk drive and restores the data in the spare disk drive in case where the number of errors occurred in that disk drive exceeds a specified value.
Further, the conventional array-type disk apparatus has an operational flow such that when a data read failure occurs frequently in a disk drive from which data is shifted (hereinafter called “data-shifting disk drive”) at the time of shifting data to the spare disk drive of the disk drive due to preventive maintenance or so, data read from the data-shifting disk drive is attempted and after a data read failure is detected, the data in the data-shifting disk drive is restored by the disk drive that has redundancy using the data restoring function of the array-type disk apparatus. It is therefore expected that the prior art drive suffers a slower response to the data read request from the host computer. To avoid the response drop, it is typical to perform the process of coping with the data read request from the host computer using only the system which isolates the data-shifting disk drive from the array-type disk apparatus when a data read error has occurred frequency in the data-shifting disk drive and restores the data in the data-shifting disk drive by means of the redundant disk drive by using the data restoring function of the array-type disk apparatus.